The subject matter relates to a temperature probe—a device placed on the skin of a subject to measure temperature. More particularly, the subject matter pertains to a deep tissue temperature (DTT) probe. Deep tissue temperature measurement is a non-invasive determination of the core temperature of a human body in which a probe is located over a region of deep tissue that is representative of the body core. The probe reads the temperature of that region as the core temperature.
A system for non-invasively measuring deep tissue temperature was described by Fox and Solman in 1971 (Fox R H, Solman A J. A new technique for monitoring the deep body temperature in man from the intact skin surface. J. Physiol. January 1971:212(2): pp 8-10). The system, illustrated in the schematic diagram of FIG. 1, estimates body core temperature by indirect means using a specially designed probe that is placed upon the skin of a subject to stop or significantly neutralize heat flow through a portion of the skin in order to measure temperature. The components of the probe 10 are contained in a housing 11. The Fox/Solman probe 10 includes two thermistors 20 mounted on either side of a thermal resistance 22, which may be constituted of a layer of insulating material capable of supporting the thermistors 20. The probe 10 also includes a heater 24 disposed at the top of the probe 10, over the elements 20, 22, and 24. In use, the probe 10 is placed on a region of the skin of a person whose deep tissue temperature is to be measured. With the bottom surface 26 of the probe resting on a person's body, in contact with the skin, the thermistors 20 measure a temperature difference, or error signal, across the thermal resistance 22. The error signal is used to drive a heater controller 30, which, in turn, operates to minimize the error signal by causing the heater 24 to provide just enough heat to equalize the temperature on both sides of the thermal resistance 22. When the temperatures sensed by the thermistors 20 are equal or nearly so, there is no heat flow through the probe, and the temperature measured by the lower thermistor 20 by way of a temperature meter circuit constituted of an amplifier 36 and a temperature meter 38 is equivalent to DTT. The probe 10 essentially acts as a thermal insulator that blocks heat flow through the thermal resistor 22; DTT probes that operate in the same manner are termed “zero-heat-flux” (“ZHF”) probes. Since the heater 24 operates to guard against loss of heat along the path of measurement through the probe, it is often referred to as a “guard heater”.
Togawa improved the Fox/Solman design with a DTT probe structure that accounted for the strong multi-dimensional heat transfer of dermal blood flow through the skin. (Togawa T. Non-Invasive Deep Body Temperature Measurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements. Vol. 1. 1979. Academic Press, London, pp. 261-277). The probe, illustrated in FIG. 2, encloses a ZHF sensor design 40, which blocks heat flow normal to the body, in a thick aluminum housing 42 with a disk-like construction that also reduces or eliminates radial heat flow from the center to the periphery of the probe.
ZHF deep tissue temperature measurement were improved in several ways, principally by decreasing the size and mass of a DTT probe to improve response and equilibrium times, and also by adding guard heating around the periphery of the probe to minimize radial heat losses. Nevertheless, ZHF probes have typically been expensive and non-disposable, and have not been widely adopted for clinical use, except for cardiac surgery in Japan. The sensors cannot be effectively heat sterilized, although they can be disinfected with a cold bactericidal solution.
Presently, ZHF probes based on the original Fox and Solman design comprise both software and hardware improvements. One such ZHF probe has a stacked planar structure that consists of a number of discrete layers. An advantage of this design is a narrow width, which helps minimize radial temperature differences from heat loss through the sides of the sensor. This probe includes an optimally-damped heater controller which is operated by use of a PID (Proportional-Integral-Derivative) scheme to maintain the heater temperature just slightly higher than the temperature of the skin. The small temperature difference provides an error signal for the controller. While the hardware design is not disposable, it does provide some basic improvements to the size and mass of the Fox/Solman and Togawa designs.
Maintenance of body core temperature in a normothermic range during a perioperative cycle has been shown to reduce the incidence of surgical site infection, and so it is beneficial to monitor a patient's body core temperature before, during, and after surgery. Of course non-invasive measurement is very desirable, for both the comfort and the safety of a patient. Deep tissue temperature measurement using a probe supported on the skin provides an accurate and non-invasive means for monitoring body core temperature. However, the size and mass of the Fox/Solman and Togawa probes do not promote disposability. Consequently, they must be sterilized after each use, and stored for reuse. As a result, use of these probes to measure deep tissue temperature may raise the costs associated with DTT measurement and may increase the risk of cross contamination between patients. It is therefore useful to reduce the size and mass of a DTT probe, without sacrificing its performance, so as to promote disposability.